In general, the superconductor is cooled to maintain the superconductor properties by means of such forced-flow cooling method or indirect cooling method as being immersed into the cooling medium such as liquid helium, or incorporating a refrigerating system. More specifically, a composite superconductor comprising a superconductor of alloy superconductor material such as NbTi or the like with aluminum is realized, using such properties of the aluminum as high specific heat, high heat transfer, adjustable electrical conductivity, small specific gravity, low radioactivity or the like (refer to the patent document 1 listed hereinafter).
However, in order to obtain highly efficient superconductor, it comes to be understood that the composite superconductor is effective, which comprises aluminum and a superconductor of compound superconductor material excellent in such superconductor properties as critical current density, critical magnetic field, and critical temperature, where thermal, mechanical, and electrical contact condition of the superconductor and the aluminum is controlled.
However, the compound superconductor material is worked to a predetermined size following drawing and rolling of the raw material including intermediate heat treatments, and then heat-treated to produce a compound superconductor functioning as superconductor. Thus produced compound superconductor is brittle to the mechanical distortion. Accordingly, the plastic working thereafter is subjected to a large constrains so as not to deteriorate the superconductor properties. More specifically, when such fabricating method as the extrusion-coating or the wire drawing is applied to the composite superconductor comprising the heat-treated compound superconductor and the metal member, in the same manner to the conventional alloy superconductor, the plastic working is applied thereto such that the critical current property is partially deteriorated, thus the above-described type of the composite superconductor is not realized yet.
Furthermore, other than the extrusion-coating method and the composite drawing method of the composite superconductor, there is known a method in which a hollow portion is formed by combining two copper members, then the compound superconductor consisting of Nb3Sn is placed in the hollow portion and then the joint portion of the copper members is soldered (refer to the non-patent document 1 listed hereinafter). Since aluminum has high heat transfer and high specific heat, it is necessary to rapidly provide large amount of heat. In addition, aluminum is promptly oxidized so that oxide layer has to be removed before soldering or brazing.
Accordingly, it is not practical to substitute the above-described copper member by the aluminum member, and then the aluminum members are jointed by soldering or brazing. On the other hand, it is also not practical to carry out the arc welding, since the arc welding (for example, TIG (tungsten inert gas) welding, or MIG (metallic inert gas) welding) has difficulty in adjustment of the heat amount imparted to the metal member during the welding so that the accuracy of the size in the joint portion is deteriorated, or the superconductor is deformed by thermal strain during the arc welding so that the critical current property is partially deteriorated.
There is known a composite superconductor comprising a compound superconductor of Nb3Al and a metal member of a tubular stainless alloy (refer to the non-patent document 2 listed hereinafter). According to the above composite superconductor, the supercritical helium is flown through the gap in the metal member so as to coercively cool the superconductor. Thus it is not possible to attain the object of the present invention in which the contact condition between the superconductor and the metal member of aluminum is to be controlled.
Patent document 1: Japanese Patent Application Publication 2000-164053;
Non-patent document 1: Cryogenic engineering 39 vol. 9, 2004, pages 383 to 390, Ando;
Non-patent document 2: Cryogenic engineering 38 vol. 8, 2003, pages 391 to 398, Koizumi